The first helical groove rotor in a barrel bore apparatus was probably used to grind meat, long before rubber was invented. The meat grinding operation wedges the process material against the barrel bore so as to cause the helical rotor groove to move the process material "enmasse" longitudinally down-stream.
The extrusion flow of the typical rubber processing extruder is quite different. Adhesion to the barrel bore and resistance to down-stream flow, causes the rubber to be moved circumferentially across the helical groove, from the leading edge to the trailing edge. The rubber arriving at the trailing edge must "push" its way downward and back across the helical groove to the leading edge. This action compresses the developed strain in the rubber and thereby is a source of non-productive work. In addition, the rubber flow pattern concentrates the mechanical shear at the barrel bore and extrusion groove surfaces and thereby tends to generate a warmed band of rubber surrounded by a cool core.
The "Pin Type" extruder, and other long extruders are actually extruders in series, separated by a dwell time section, long enough to blend the unequal temperature generated by the helical extrusion grooves. The extrusion therefore must be a blend of over-worked and under-worked process material.
The "Transfer Type" extruder which transfers the process material from rotor to barrel to rotor is the best of present day extruders. However, the transfer of process material from rotor to barrel tends to wind the warmed process material into a spiral roll in the receiving barrel groove. This roll, reaching the barrel to rotor transfer, tends to un-wind as it is transferred back to the rotor. Although this is an over simplification of the actual transfer, there is room for improvement.
The "Barrier Type" extruder, which provides an extended length barrier across the helical extrusion groove, accelerates the flow of the fluent phase process material while retarding the less fluent and oversize material. This is particularly effective on small size extruders, as the clearance barrier top to barrel bore can also be small. The "Barrier Type" extruder is widely used in plastic film extrusion. It is a means of providing equal processing to the process material.
All extruders tend to operate on the first in first out basis, similar to the flow of liquid through a pipe. This action limits extrusion blending, as practiced today, to cross-section blending of the process material. The present invention therefore provides two different means of longitudinal blending.
In present day mill room equipment, the internal mixer is provided with geometry which shears the baled polymer to size, introduces and incorporates powdered and liquid chemicals up to one half the weight of the polymer and discharges the resulting material as a non-uniform batch. (Occasionally loose black is part of the drop). To blend longitudinally the internal mixer is followed with the mills to complete the mix. Mills, the back bone of the rubber industry, in addition to other problems, are not an efficient mixer. The mill forms a band of process material on one roll. This band at the roll surface adheres to the roll and receives little shearing. The outer portion of the band is worked and elongated as it passes through the mill nip. The elongated material, returning to the mill nip must "push" its way in and thereby loosen the previously developed strain.
Mills equipped with an over-head blender can blend the batch to acceptable levels. However, equipment, power and a mill man are required. A minimum of two mill men in the area are required for safety purposes. In extrusion mixing as disclosed in U.S. Pat. No. 4,929,086, an apparatus (which replaces the 1,000 unit batch of the internal mixer with a 1,000 one unit batch) eliminates the need of post mixer mixing. Other examples of the prior art are the following U.S. Pat. Nos. 2,744,287 (Feb. 23,1954 ); 3,375,549 (April 1968); 3,888,469 (Jun. 10, 1975); 4,075,712 (Feb. 21, 1978); and 4,872,761 (Jul. 31, 1990).
Although many problems relative to the extrusion of plastic materials have been solved, temperature stratification by helical extruder grooves, longitudinal blending of the process material and mixing with minimum work input are continuing problems.